HF tunning

[Gary Schafer]

Bob

Depends on what you are driving it with for am. If you are using an ssb
rig to go on am with, tune the amp for max output in the cw mode just
like you would tune up for ssb operation. Leave everything alone and
switch to am. Set the am carrier level for 1/4 the power OUTPUT on the
amp that you got on tune up. Adjust the audio gain so your alc just
wiggles once in awhile. That should get you in the ball park. Your amp
needs to be loaded for 4 times the output of the carrier on am as the
peak envelope power on am is 4 times the carrier power.

If you are using a regular am transmitter (none ssb) to drive the amp on
am you will need a peak reading watt meter or a scope to tune up. You
need to modulate the transmitter 100% with a tone or whistle or voice
(long heeeeeloo) while tuning amp for max peak output. Again, when you
are done the carrier out should be 1/4 of what the peak power out is. If
using a scope it will show 1/2 the voltage for the carrier that it
showed on max peak output.

The efficiency of the amp with carrier when properly tuned for am will
run about 33% (input power to output power)

If you have "negative mod" or a downward kick in plate current you have
too much carrier. It could be that the amp is tuned ok but the exciter
is not loaded properly also. All linear stages (including the exciter)
need to have that 4 to 1 ratio of peak power to carrier for proper am
operation. Regardless of what the manual may say!!

This is not entirely true though for some of the newer rigs where the
alc is used to reduce the carrier if the modulation ratio is not 4 to 1.
It kind of works like reverse controlled carrier.

73
Gary  K4FMX


Bob Meyer wrote:

> I have a few HF amps here.When tunning ,use 2 tone ssb to peek plate
> and ant.on one manual I have here sayes to dummy load tune firstand go
> to ant.That is what I do.FM/cw just peek plate and load.with the
> ken-tron amp,for AM peek and back off load(less cap) 3 to 5%.I found I
> have to with that amp because peeking am willcause neg mod.drooping
> load off makes AM cleaner. I was woundering on heath-kits and outherHF
> amps using 811A,3-500Z and outher tubesshould tune AM the same way??
> Robert

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